Conventionally, there are some optical instruments such as bar-code readers or projectors that a micro-mirror device is mounted thereon to scan light beams by swinging a moving plate to which a mirror is provided and reflecting the light beams incident to the mirror to desired direction exist. Such a micro-mirror device is one to which a miniature moving structure formed by so-called micro-machining technology is mounted, and a moving plate is held by a pair of beam shaped hinge units supported by circumferential frame unit. The moving plate receives driving forces such as magnetic forces or electrostatic forces, and swings with twisting the hinge units using the hinge units as a rotation axis.
By the way, in the above mentioned moving structure, there are five kinds of “torsion mode”, “in-plane translation mode”, “in-plane turn mode”, “out-of-plane translation mode” and “out-of-plane turn mode” in vibration modes of the moving plate, as shown in FIG. 4. In order to scan the light beams precisely, it is necessary to control the posture of the moving plate precisely, and it is demanded to increase response and stability of swing motion of the moving plate. In order to increase the response of the swing motion of the moving plate, it is necessary to design the hinge units to set the resonance frequency in the “torsion mode” to be lower than a constant level, so that various kinds of shapes which enable to reduce torsional rigidity of the hinge units are considered.
For example, a patent document 1 discloses a technology to form a cross-sectional shape of the hinge units to “X shape”. In addition, a patent document 2 discloses a technology to form the hinge units by two supporting rods which are obliquely disposed to face each other with a space,